An insulated gate bipolar transistor (hereinafter referred to as IGBT) 80 shown in FIG. 9 is known as a semiconductor device having a planar type high voltage-proof vertical element. The IGBT 80 is a semiconductor device which has both a high input impedance characteristic observed in a metal oxide field effect transistor (hereinafter referred to as MOSFET) and a low saturation voltage characteristic known as a characteristic of a bipolar transistor.
A substrate 82 used for the IGBT 80 includes a drain layer 3 with P+ type, an n+ type layer 5 and an n type layer 7. Base regions 21 are formed in the n type layer 7, and source regions 23 with n+ type are formed within the base regions 21. The surface of the n− type semiconductor layer 82 is covered with a gate oxidation layer 22.
Incidentally, a loss caused by switching arises as a result of a parasitic diode generated on a plane of a PN (positive-negative) junction 59. Japanese Patent laid-open publication No. Hei 7-135214 discloses a technology for selectively radiating electron-beams using a mask 41 shown in FIG. 9 during the manufacturing processes in order to avoid the generation of the parasitic diode. The beams pass through through-holes 43 formed in the mask 41 and irradiate on the IGBT 80. In this way, the life-time of carriers located on the plane of the PN junction 59 where the parasitic diode being generated can be shortened as a result of forming crystal defects 61.
In the manufacturing processes described above, unexpected variation of the threshold voltage in the IGBT 80 is possible because of generation of bremsstrahlung caused by the material of the mask 41 made generally of a heavy metal such as lead and the like.
Japanese Patent laid-open publication No. Hei 8-227895 discloses another IGBT 90 having layers for restricting electron-beams. As shown in FIG. 10, a layer 69 made of silicon nitride for restricting the beams is formed under a source electrode. Generation of bremsstrahlung is restricted even when the beams are radiated to the IGBT 90 due to masking by the restriction layer 69.
However, the number of processes is increased because the processes for forming the restriction layers 69 made of silicon nitride is required in the conventional process.